1. Field of the Invention
The present invention relates to a method for setting search band widths in a GPS receiver.
2. Description of Background Information
A Position measuring system using artificial satellites is known as Global positioning System (GPS). The system employs six orbits set around the earth at an altitude of about 20200 km. Into each of the orbits, a plurality of satellites are put up so that GPS signals including navigation data for the position measurement are transmitted from the satellites toward the earth by a spread spectrum method. The radio waves of three or four satellites which are necessary for the position measurement are received by a GPS receiver on the ground (or on the sea or in the air), thereby enabling the calculation of necessary position information such as latitude, longitude, altitude of the reception point in a real-time manner by using the navigation data included in the GPS signal from each satellite.
The GPS signal which is released for non-military use is called C/A code, and is transmitted at a frequency of 1575.42 MHz. Therefore, to capture the satellite signal, in principle, it is sufficient to wait for the reception of the radio wave at 1575.42 MHz in a standby mode. However, since the GPS satellite is not a geostationary satellite, fluctuation of the received frequency will occur due to a Doppler effect. Furthermore, the oscillating frequency of an oscillator of the GPS receiver also fluctuates due to an offset of such an oscillator.
To capture the satellite signal in practice, accordingly, a search center frequency of the satellite radio wave is set by a frequency shift corresponding to a deviation amount of the received frequency due to the Doppler effect (hereinafter, referred to as "Doppler frequency deviation") and an offset amount of the oscillator. Furthermore, an appropriate search bandwidth is set, so that the satellite signal is searched for while shifting the search frequency in the high and low frequency directions within the search bandwidth, with a search start point being set to the search center frequency. Thus the capture of the satellite signal is completed at a point in time when the oscillator is locked to the satellite radio wave. By repeating the above capturing operations, signals from the satellites of the number necessary for the position measurement are captured.
In the operation of capturing the satellite signal, the narrower the search bandwidth the shorter the capturing time of the satellite signal. Also, the smaller the offset value of the oscillator, the narrower the search bandwidth. Therefore, in order to set the search bandwidth as narrow as possible, it is desirable to minimize an error of the offset value of the oscillator of the GPS receiver.
In conventional GPS receivers, accordingly, a crystal oscillator with a temperature compensation device of an extremely high accuracy (hereinafter, such an oscillator is referred to as "TCXO") is used as an oscillator. Furthermore, those receivers are designed to store into a memory the offset value of the oscillator calculated last when the power source of the receiver is turned off upon completion of the position measurement, as a backup value. The backup value is used as an initial offset value of the oscillator at the beginning of the next position measurement.
The TCXO exhibits, by the compensation of temperature characteristics of the quartz resonator by an electrical circuit, an extremely high accuracy of about .+-.2 to .+-.5 ppm (parts per million) (.+-.3 kHz to .+-.8 kHz in the frequency) in a wide temperature range from -40.degree. C. to +85.degree. C. On the other hand, the accuracy of oscillators used in an ordinary communicating apparatus lies within a range of about .+-.50 to .+-.100 ppm and, preferably, about .+-.20 ppm.
In conventional GPS receivers, since the highly accurate TCXO is used, there has been no need to consider the change in offset value of the oscillator due to a temperature change in the receiver for a period of time when the power source of the receiver is OFF. The offset value backed up in the memory is directly used as an initial offset value at the start of the next position measurement. In the event that the data stored in the memory is extinguished or the backup data is made invalid due to an error of the memory or the like, the search range is determined as wide as the whole fluctuation range of the offset value of the TCXO.
If an oscillator of a low accuracy is used, on the other hand, a change in offset value due to a temperature change is large and the offset value backed up in the memory cannot directly be used as an offset value of the oscillator at the start of the next position measurement. Therefore, when an oscillator of a low accuracy is used, it is necessary to search for the satellite radio wave in a wide band over a whole range of the offset change of the oscillator. A problem arises in that a long time is required to capture the satellite signal because of the necessity of the search through such a wide band. Such a problem also occurs even if a backup value of the offset value is used.